Such facilities are necessary, for example, where predetermined data sequences are to be adapted to different processing clocks while the signal contents proper are to remain unchanged. Such applications relate particularly to oversampled audio signals, but also to video and other signals whose sampling rate has to be reduced by a noninteger factor for further processing or storage. The invention can also be used where the signal content is to be changed. In the case of video signals, for example, the invention permits the reproduction of an image of reduced size. By reducing of the number of data while maintaining the data rate, an image of reduced size can be displayed in a window of a television or computer screen. The resulting image may be, for example, a still image, a moving television image, or a computer-controlled animation.
In general, relatively complex interpolation filters with transversal and/or recursive filter stages are used for such applications to permit a reduction of the data rate by a noninteger factor. Such sampling-rate converters are described, for example, by Tor A. Ramstad, "Digital Methods for Conversion Between Arbitrary Sampling Frequencies", Transactions on Acoustics, Speech, and Signal Processing, Vol. ASSP-32, No. 3, June 1984, pages 577 to 591.
The facilities for reducing the data rate are relatively easy to implement if the old and new data rates differ by only one sample value in successive equal sampling intervals, for example if two new samples are to be formed from three successive samples--this corresponds to a data rate reduction factor of r=1.5--or if nine new samples are to be interpolated from ten old samples--this corresponds to a data rate reduction factor of r=10/9. For these specific data rate reduction factors, very simple filter devices are known in which one sample is omitted at regular intervals and the remaining data sequence is smoothed with a digital low-pass filter.
Particularly simple circuits and methods are known if the data rate is reduced by an integer factor r. In that case, groups of data values which form a data window are added together to form a new data value, and the sequence of new data values then forms the desired new data sequence. In digital signal processing, this type of sampling rate reduction is also referred to as decimation. The data values in the data window may be modified according to the selected interpolation method by an associated weighing function. In view of the simplest weighing case, in which the data within the data window are not weighted, these circuits are also referred to as "comb filters". They are described in an article by Shuni Chu and C. Sidney Burrus, "Multirate Filter Designs Using Comb Filters", IEEE Transactions on Circuits and Systems, Vol. CAS-31, No. 11, November 1984, pages 913 to 924.
Such comb filters generally comprise at least one accumulator, one decimation switch which is closed after the predetermined number of samples, and one differentiator. Each accumulator contains a delay stage and an adder, and each differentiator contains a delay stage and a subtracter. The adders and differentiators together form a tracking data window for forming the interpolated sample value. A single accumulator does not perform a weighing function. With two or more series-connected accumulators and an equal number of series-connected differentiators, however, even higher-order weighing functions can be implemented. If the last accumulator is designed as a resettable accumulator which is reset together with the decimation switch, the number of differentiators required will be reduced by one. The comb filter structure also has the advantage that a high processing clock frequency is only necessary for the accumulators, while the differentiators operate at the lower clock frequency of the decimation switch.
It is an object of the invention to provide a simple circuit for reducing the data rate by a noninteger factor which can be represented as a rational number.